1. Field of the Invention
This invention relates to an oscillatable marine installation and, in particular, a platform to be installed on a permanent site in the sea or other expanse of water which can have various uses, such as for storing materials or carrying drilling equipment.
The installation comprises a shaft resting on a base fixed to the sea bed, which supports on its upper part the so-called platform or bridge, the height of the shaft being sufficient for the bridge to be always above water-level.
The shaft is connected to the base by means of a spherical joint which enables it to oscillate in all directions as it is moved by the swell, and is ballasted near its base in such a way that the total weight of the ballast, shaft, bridge and the load carried in greater than the maximum upward thrust due to buoyancy and to the action of the elements, whereby the spherical joint is always compressed.
2. Description of the Prior Art
A similar construction is known from U.S. Pat. No. 4,126,010, in which the shaft comprises a watertight hollow column which can receive ballast at its lower end, while the upper part comprises a float. Such an invention is particularly suitable for an installation of relatively low height. However, the greater the height, the greater is the tendency for the shaft to respond to the swell. In order to avoid this disadvantage, elements of varying rigidity are positioned at intervals.
The float part of an installation with a cylindrical shaft is particularly vulnerable near the surface since there is the risk that it may be damaged by boats which are being moored. In spite of the float being divided into compartments, any leakage of water into a compartment causes the installation to list considerably, to a degree which cannot be accommodated by drilling pipes. The movements of the platform subject the drilling pipes, which are generally positioned on the periphery of the shaft, to bending stresses which increase with their distance from the axis of the shaft, and even more so if the distance separating the bottom of the shaft from the base is small.
U.S. Pat. No. 3,670,515 discloses a solution to this problem. It describes a rocking drilling platform comprising a shaft composed of a grid type of structure having a float on its upper part. The lower end of the shaft is connected to the base, which is held on the sea-bed by means of piles, by a joint. The drilling pipes pass downwardly along the shaft and then to the exterior thereof where they are guided by supports. In order to avoid too much bending of the pipes to one side of the joint, the latter is supplied with a support device in the plane of the joint, this device being connected to the shaft and to the base by articulated rods which enable the pipes to slide freely. Such an arrangement necessitates sufficient free lengths of pipe so that the loads can be distributed and so as to avoid exceeding the elastic limit of the metal.